The detection of C\(_2\) H\(_4\)  was first reported by [1]} toward IRC+10216.
A mid-IR observational study of C\(_2\) H\(_4\)  toward IRC+10216 by [2]} revealed that the C\(_2\) H\(_4\)  abundance relative to H\(_2\)  is between \(10^{-8}\)  and \(10^{-7}\) .
From the view of top-down chemistry, H\(_2\)  formation from PAHs does not deplete carbon atoms in PAHs so that PAHs can be repeatedly used to generate H\(_2\) .
C\(_2\) H\(_4\)  loss from PAHs must take away carbon atoms from PAHs, thus following C\(_2\) H\(_4\)  formation is prohibited due to the depletion of carbon atoms.
In addition, C\(_2\) H\(_4\)  formation is more sensitive to hydrogenated sites of PAHs than H\(_2\)  formation.
It is expected that H\(_2\)  formation has more viable routes than C\(_2\) H\(_4\)  formation via top-down chemistry of PAHs.
